1. Field of the Invention
Embodiments of the present invention relate generally to an apparatus for displaying three-dimensional (3D) images, and more particularly, to a two-dimensional/three-dimensional (2D/3D) switchable autostereoscopic display apparatus.
2. Description of the Related Art
3D display technologies are classified into so-called stereoscopic display technology that requires viewers to wear special glasses such as liquid crystal shutter glasses to view 3D stereoscopic images, and so-called autostereoscopic display technology that does not require the wearing of glasses.
Devices constructed with stereoscopic display technology can employ liquid crystal shutter glasses, in which left- and right-eye liquid crystal shutters alternately pass and block light during predetermined periods, and in which a shutter glass driving device drives the shutters. Stereoscopic images may be recognized by providing left- and right-eye images independently after separating them. However, stereoscopic display technology has many shortcomings, in particular requiring liquid crystal shutter glasses and their associated devices such as shutter glass driving devices.
Devices employing autostereoscopic display technology have the advantage of displaying 3D images without requiring the viewer to wear shutter glasses. Autostereoscopic display technology may employ a parallax barrier 3D display device or a lenticular 3D display device. The parallax barrier 3D display device has a parallax barrier with apertures in the form of a vertical grid, which is mounted in front of a display panel having pixels arranged in rows and columns. The parallax barrier separates left-eye images for the viewer's left eye from right-eye images for the viewer's right eye, and causes binocular disparity between the different images on the display panel. On the other hand, to display 3D images, the lenticular 3D display device generally uses a lenticular lens having a columnar (or vertical) array of semi-cylindrical lenses, which is put on the display panel, instead of using a parallax barrier.
A 2D/3D switchable lenticular 3D display device includes a display panel and a liquid crystal lens that serves as a lenticular lens depending on the change in refractive index of a liquid crystal material filled between upper and lower substrates. The liquid crystal lens is mounted in front of a 2D display panel, and may switch between 2D and 3D display modes depending on a voltage applied between electrodes thereof.
In the 2D display mode, the voltage applied between or across the electrodes is turned off, i.e. not applied, or its application is stopped. Then, the liquid crystal between the electrodes may maintain its initial alignment state, so the liquid crystal lens no longer serves as a lenticular lens and the light coming out from the 2D display panel is provided to the viewer as 2D images.
In the 3D display mode, the voltage applied between or across the electrodes is turned on. Then, the liquid crystal lens may serve as a lenticular lens by changing the refractive index of the liquid crystal differently depending on the location of the liquid crystal material. As the liquid crystal lens serves as a lenticular lens, the light coming out from the 2D display panel undergoes a change in its propagation direction while passing through the liquid crystal lens, and provides different images to the viewer's left and right eyes, making it possible for the viewer to view stereoscopic images.
In order for the liquid crystal lens to precisely control the images provided to the viewer's left and right eyes in the 3D display mode, polarizers are mounted on the upper and lower substrates. The polarizers absorb the light that proceeds in undesired directions, making it possible to control the direction of the light passing through the liquid crystal lens.
Disadvantageously, however, these polarizers may reduce luminance of the light passing therethrough. A light absorption loss of one polarizer is currently about 7%. Therefore, the polarizers included in the liquid crystal lens are significant causes of luminance reduction.
When a 3D display device uses a liquid crystal display panel as its display panel, an additional luminance loss may occur as follows. The liquid crystal display panel adjusts brightness of the light passing therethrough using two polarizers. The liquid crystal display panel transmits only the light linearly polarized in the vertical or horizontal direction using the polarizers. In the conventional liquid crystal lens, liquid crystal molecules are aligned in an anti-parallel way (e.g., aligned parallel but in opposite directions), and polarizers attached to its upper and lower substrates have their polarization axis in the same direction as the alignment direction of the liquid crystal material.
In order to prevent color separation and moire phenomena which may occur in 3D images, an array direction of liquid crystal lenses is required to be tilted with respect to the pixels arranged in the vertical (or columnar) direction. However, when a polarization direction of the polarized light passing through the 2D display panel is inconsistent with a direction of a polarization axis of polarizers mounted on the liquid crystal lens, a λ/2-phase difference film is required to match them with each other. The λ/2-phase difference film rotates the polarized light output or projected from the 2D liquid crystal display panel as required in the liquid crystal lens. However, when the polarization axis of the transmitted light is rotated by φ by means of the λ/2-phase difference film, a light loss caused by the rotation of the polarization axis may occur, as defined by Equation (1) below.Polarization Axis Rotation Loss=1−cos2φ  (1)where φ denotes an angle by which the polarization axis is rotated by the λ/2-phase difference film, and cos2φ denotes a transmittance given when the polarization axis is rotated. For example, for φ=18.5°, the polarization axis rotation loss may be about 10%.
In addition, the polarizers are very expensive. Therefore, a plurality of polarizers used for the 3D display device may increase the total price of the 3D display device to an undesirable degree.
Therefore, there is a need to provide a 2D/3D switchable display apparatus that can reduce the number of polarizers used in the liquid crystal lens and improve its luminance.